In a recent highly information-oriented society, a storage device temporarily or semipermanently holding a large amount of information has been indispensable. Above all, a dynamic memory (DRAM), a flash memory, a read-only memory (ROM) and the like for use in a computer are well known.
A first related art is a flash memory. A storage cell of the flash memory comprises one floating gate type transistor. A floating gate electrode disposed between a channel region between source and drain and a control gate electrode is used as a storage node of information.
A charged state of the floating gate electrode is set in accordance with “0” and “1” of the information. Since the periphery of the floating gate electrode is surrounded with an insulating film, the charge stored in the electrode is not lost even after power cut-off, and therefore, nonvolatility is realized. A read operation is performed by using a property of a threshold voltage which changes in accordance with a charge amount stored in the floating gate electrode. A write/delete operation of the information is performed by injection of electrons into a floating gate or release of the electrons from the floating gate electrode by a tunnel current via an oxide film.
A second related art is a quantum point contact switch using an electrochemical reaction in the solid electrolyte (refer to Riken Review No. 37, p. 7, 2001). The solid electrolyte is a material in which ions are freely movable in a solid as in a solution, and many materials that exhibit conduction of cations or anions have heretofore been found. When an electric field is added, metal ions constituting carriers move in the solid thereby to flow currents.
In the above-described document, a switch using silver sulfide which is a silver ion conductive solid electrolyte is described. The surface of a silver wire is sulfurated to form silver sulfide, and a platinum wire is brought close to a micro gap. When a positive voltage is added to silver sulfide, and a negative voltage is added to platinum, silver ions in silver sulfide are deposited as silver atoms on the surface, and a bridge of silver is formed in the gap from platinum to form a point contact. The current hardly flows between silver sulfide and platinum in case where the bridge is not formed. When the bridge is formed, the current flows.
The formation and disappearance of the bridge occur at a high rate which is a microsecond or less. The current flowing in the bridge is quantized. The quantization of the current indicates that the bridge is formed of several atomic chains, and has a size of a nanometer order. With the use as the switch, a high-rate operation, low power consumption, and further high integration are possible. In the related art, it is described that application to the switch or memory results in formation of a new device.
The flash memory of the first related art is a device characterized by a low bit cost, and it is necessary to realize an advantage bit cost with respect to another memory. In order to realize this, scaling of a storage cell is supposed to advance from now on. However, prospects are not bright in the actual situation. One of causes for that lies in a tunnel oxide film leak current generated with an increase of the number of rewrites.
The leak current is a fatal phenomenon in which the charges stored in the floating gate electrode are eliminated. Since the leak current rapidly increases with the decrease of thickness of the oxide film, it is supposedly difficult to reduce the thickness of the tunnel oxide film. There arises a necessity for consideration of the scaling which does not depend on the film thickness reduction.
In the second related art, upon forming the gap, use is made of a method of using a scanning type tunnel microscope and a method of manually bringing two metal wires close to each other. The method of using the scanning type tunnel microscope has an advantage that one gap can be formed with excellent controllability, but is not suitable for forming a large number of gaps. The method in which the silver sulfide wires or the platinum wires are manually brought close to each other is deteriorated in controllability, and is similarly inappropriate for forming a large number of gaps. Furthermore, in silver sulfide on a silver wire as in the second related art, a size of one storage cell is of a millimeter order, and is not suitable for integration. Therefore, the integration of a storage device is impossible.
It is therefore an object of the present invention to provide a storage device using a solid electrolyte, particularly to a storage device having a circuit constitution advantageous for integration and a method of manufacturing the device.